Broadband tunable in-line filter for fiber optics

ABSTRACT

A broadband tunable in-fiber filter includes a grating with divergent ridges which can be translated transversely of a side-polished optical fiber to vary the periodicity at an exposed evanescent field. The divergence is gradual so that at any given transverse position of the grating, the ridge interacting with the evanescent field are effectively parallel. The divergence is great enough so that a tuning-to-reflected bandwidth ratio of about 33:1 is demonstrated. The grating is fabricated in an amorphous silicon film on a fused quartz substrate. The film is coated with photoresist which is exposed to a holographic interference front. The substrate is tilted with respect to an interference front created by two spherically diverging beams to achieve the desired divergence. Subsequent processing, including etching are standard.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 07/355,144, filed May 15,1989 now U.S. Pat. No. 4,986,623 which is a continuation of applicationSer. No. 07/004,997 filed Jan. 20, 1987 (now abandoned).

BACKGROUND OF THE INVENTION

The present invention relates to fiber optics, and, more particularly,to filters for fiber optic systems.

Filters for selecting a narrow frequency band from a relatively broadband of transmitted electromagnetic radiation, "light" herein, have manydiverse applications. For example, such filters can be used to providenarrower bands of light than might be otherwise available from a laseror other source. In one application, light so filtered can be reflectedback to a laser source to tune the laser to a desired band.

Fiber optic filters can also be used to improve signal-to-noise ratiosby isolating a signal carrying band from adjacent frequency bands. Also,selective filters can be used to demultiplex wavelength divisionmultiplexed (WLDM) optical signals. A diverse array of other uses can berecognized by analogy to electronic systems.

Until recently, filters for fiber optics used bulk optic approaches inwhich light was removed from an optical fiber to be filtered externally.The filtered light could then be reintroduced into a fiber forsubsequent transmission.

One disadvantage of the bulk optic approaches is the bulk of theexternal filter and required couplers. Another disadvantage relates tothe necessarily tight mechanical tolerances and resultant vulnerabilityto vibration and environmental changes. The complexities and lossesinvolved in the bulk optic approaches have inspired a search for aneffective way to filter light "in-line", i.e., without removing it fromthe fiber. Devices which function by interacting directly with theevanescent fields of guided light in a single mode fiber are thus ofconsiderable interest as elegant and compact alternatives to bulk opticdevices.

An approach to "in-fiber" filtering, applicable to single-mode fibers isdisclosed by W. V. Sorin and H. J. Shaw in "A Single-Mode FiberEvanescent Grating Reflector", Journal of Lightwave Technology, Vol.LT-3, No. 5, October 1985, pp. 1041-1042. This article disclosed the useof a metal diffraction grating disposed upon the side-polished portionof an optical fiber. This arrangement provides narrow band reflectionsof light transmitted from a laser source.

The evanescent trailing fields in the neighborhood of the fiber core canbe reached using a well-known technique in which the fiber is strippedof its coating, embedded with epoxy in a groove cut in a glass block,ground down to the vicinity of its core and highly polished. Thistechnique is described in a disclosure by R. A. Bergh, G. Kotler, and H.J Shaw, entitled "Single-Mode Fiber Optic Directional Coupler",Electronics Letters, Vol. 16, No. 7, Mar. 27, 1980, pp. 260-261.Interaction between the diffraction grating and the evanescent field isobtained by placing the metal surface of the grating in contact with thepolished portion of the fiber. An index oil can be applied between fiberand grating to remove air gaps which would otherwise decrease the extentof the evanescent field.

Herein, a fiber prepared according to the above technique is referred toas a "side-polished fiber". The region so-polished is referred to as the"side-polished region" of the fiber.

The grating used by Sorin and Shaw includes a series of parallel ridgesperiodically disposed on a pitch of 0.278 microns (μm). The grating forsuch a filter can be fabricated using well-known holographic techniques.The interference front generated by an intersecting pair of collimatedlaser beams can produce a series of generally parallel interferencelines. These lines can be used to expose a photoresist coated substrate.The exposed substrate can be processed so that the interference patternis represented as ridges on the finished grating.

While subsequent analysis has shown that many of the advantages soughtfrom in-fiber filters are obtained by the device disclosed by Sorin andShaw, it compares unfavorably with external filters in one importantrespect: heretofore, broadband tunable in-fiber filters have not beenavailable. In contrast, external filters using bulk optical elementshave been provided which are tunable over a broad band. Herein, "broadband" and "broadband" are used to refer to tuning ranges which are largerelative to the bandwidth being tuned.

In one bulk optic reflective filter approach, light is removed from anoptical fiber and collimated using an optical lens. The collimated lightis then directed against a bulk optic diffraction grating. Thediffraction grating diffracts the incident light so as to reflect anarrow band of wavelengths back along the direction of the incomingcollimated beam. The reflected band is then coupled back into the fiber.

In this bulk optic approach, tuning is achieved by tilting the bulkoptic grating, thereby changing the wavelength back towards the fiber.Rotating the grating changes its effective spatial periodicity in thedirection of the incident light. The spatial periodicity determines thefrequency band which is superimposed constructively back along thevector of incidence.

As indicated above, such bulk optic approaches are disadvantageous inrequiring stringent mechanical tolerances since light must be coupledback into the fiber whose mode diameter can be less than 10 μm.Furthermore, considerable space is required for the arrangement of thebulk optic components. Accordingly, tunable in-line filters are desired.

Heretofore, the tunability of in-line fiber optic filters has been verylimited. The filter disclosed by Sorin and Shaw can be tuned by rotatingthe grating relative to the fiber. This increases the spatialperiodicity of the grating in the direction of the fiber. However, asthe grating is rotated, the orthogonality of the ridges to the directionof propagation is diminished so that the quality of the reflected signalis impaired. Thus, the tuning range is practically limited to abandwidth comparable to the bandwidth of the reflected signal.

Two other approaches for tuning a grating filter are disclosed by C. A.Park et al., in "Single-Mode Behavior of a Multimode 1.55 μm Laser witha Fiber Grating External Cavity", Electronics Letters, Vol. 22, No. 21,Oct. 9, 1986, pp. 1132-1133. The spatial periodicity of the grating canbe increased by heating. However, the tuning was limited to 13 angstroms(Å) relative to a transmitted wavelength of 15,620 Å. The tuning rangewas just slightly larger than the reported 10 Å reflection bandwidth. Aneven weaker tuning effect was achieved by varying the refractive indexof oil placed between the cladding and the grating. In addition to thelimited tuning ranges provided, a disadvantage of the temperature andoil approaches are the impracticality of varying these parameters,especially over the ranges required to obtain greater than narrow-bandtuning.

Thus, the tuning range of disclosed in-line fiber optic filters has beenlimited to less than twice the reflection band of the filter ofinterest. However, in many applications a tuning range at least an orderof magnitude greater than the reflection bandwidth is desired.

The quest for a broadband tunable in-line fiber optic filter faces twomajor challenges. The first is the determination of a structure that canprovide the desired tuning function. The second is determining a methodof manufacturing a device with the required structure, given thedimensions and precision required in fiber optical systems. Both thesechallenges are met as described below.

SUMMARY OF THE INVENTION

The present invention provides for an in-line fiber optic fiber filterincorporating a movable grating in which pitch varies as a function ofposition. By moving the grating relative to the side-polished region ofan optical fiber, the evanescent field of light being transmitted alongthe fiber optic core can interact with different regions of the gratingcharacterized by different pitches. The selected pitch determines thefrequency band of the light reflected along the fiber.

In a preferred embodiment of the present invention, the grating includesdiverging ridges. The divergence is gradual enough so that the ridgesare substantially parallel to the extent they extend transversely overthe side-polished region of an optical fiber. More specifically,substantial parallelism applies to ridge segments defined by ahalf-power interaction region defined by an intersection of anevanescent field with the plane of the grating. On the other hand, theproduct of the divergence and the transverse extent of the grating isgreat enough so that the pitch at the side-polished region can be variedto produce a significant change in reflected frequency.

The grating for the filter can be manufactured using a modification oftechniques used in fabricating gratings with parallel ridges. However,instead of orienting the grating substrate orthogonal to a holographicfront generated by collimated beams, the substrate is tilted withrespect to an interference front generated by spherically divergingbeams so that the interference lines at the substrate diverge.

Tests on the preferred filter demonstrate a reflection efficiency of 65%at 13,000 Å. The measured tuning range is 260 Å, which comparesfavorably with the measured reflected bandwidth of 5.0 to 6.0 Å.

In the preferred embodiment, the tuning range is at least an order ofmagnitude greater than the reflection bandwidth attainable. Thus, abroadband tunable filter for a fiber optic system is provided withoutthe complexities and losses inherent in removing and coupling the lightto take advantage of external filters using bulk optics.

More specifically, in comparison to tunable bulk optic filters, thepresent invention provides a filter which requires less space, is moremechanically rugged, has more relaxed mechanical tolerances and isoptically more efficient. Regarding fiber coupling tolerances, the fiberreflector is more than two orders of magnitude less sensitive to angularmisalignment than a comparable bulk optic device. This yields greatlyincreased stability in the face of vibrations and environmental changes.

Concurrently, the tuning range is much greater than heretofore providedby in-line fiber optic filters. Furthermore, the tuning is accomplishedwithout comprising the orthogonality of the ridges: thus, the quality ofthe reflected signal is not impaired as it is in the rotated gratingapproach to in-line filtration. No hydraulics are required, in contrastto approach using oils with different refractive indexes. Furthermore,the side-effects of large temperature changes, used in the thermalexpansion approach to tuning, are avoided. Other features and advantagesare apparent in view of the detailed description below in conjunctionwith the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a filter for a fiber opticsystem in accordance with the present invention.

FIG. 2 is a schematic perspective view of a subassembly of the filter ofFIG. 1.

FIG. 3 is a sectional side view of the subassembly of FIG. 2.

FIG. 4 is a schematic plan view of a grating for the filter of FIG. 1shown in relation to an optical fiber.

FIG. 5 is a schematic view of an apparatus used in manufacturing agrating for the filter of FIG. 1.

FIG. 6 is a graph depicting reflected wavelength as a function ofgrating position for the filter of FIG. 1.

FIGS. 7A, 7B and 7C are graphs showing the spectral distribution oflight reflected by the filter of FIG. 1, with the grating of the filterin three respective positions.

FIG. 8 is a schematic illustration of an alternative grating providedfor by the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a filter 10 for a fiber opticsystem is shown comprising a side-polished optical fiber 11, a block 13for retaining the fiber, a grating 15, and a translation stage 17 formoving the grating relative to the fiber, as shown in FIG. 1. Thegrating 15 has gradually diverging ridges 19 which extend substantiallytransversely of the fiber 11, as best shown in FIG. 2. The filter 10 istuned by operating the Y-control 21 of the translation stage 17 so thatthe spatial periodicity of the grating 15, i.e., the pitch of the ridges19 over the side-polished region 23, is changed.

The preferred fiber 11 is a single-mode fiber. Such a fiber provides forprecise control of transmitted light since the single propagated modecan be operated on with negligible side-effects due to other propagationmodes.

The preferred grating 15 is a first-order grating, i.e., its spatialperiodicity is nominally half the wavelength of the mode to bereflected. By using this first-order grating, higher order diffractionmodes, which would not be captured by the single-mode fiber 11, areeliminated.

The fiber 11 includes a core 25 surrounded by a cladding 27, as shown inFIG. 3. The fiber 11 is shown fixed in position within an arcuate groove29. At the center of the side-polished region 23, the cladding 27 isthinned insofar as possible without breaching the integrity of the core25. Preferably, the minimum thickness of the cladding 27 at itsside-polished region 23 is less than or at most comparable to thediameter of the core 25.

The purpose of this side-polished region 23 is to permit the evanescentfield of incident light P_(IN) being transmitted along the core 25 toprotrude from the fiber 11 to interact with the grating 15 to produce areflection P_(R). The intersection of such an evanescent field with thesurface of the cladding 27 is contained within the side-polished region23, which thus can be used to roughly locate the exposed evanescentfield. Specifically, the region of interaction of the evanescent fieldwith the grating is generally confined to the orthogonal projection ofthe side-polished region 23 onto the grating 15. Several projectionregions A, B, C, D, E and F, corresponding to different transversepositions of the grating 15, are indicated in FIG. 4.

However, a more precise definition for a region of interaction at thegrating is available. For a given grating position, at some point on thegrating, the evanescent field of light being transmitted along the coreachieves maximum strength. A "half-power interaction region" can bedefined as that region in the plane of the grating in which theevanescent field is half or more of its maximum. Herein, a region of thegrating is said to substantially contain the intersection of the gratingand the evanescent field when the region contains the half-power regionof the evanescent field defined in the plane of the grating.

This half-power interaction region is much smaller than theside-polished region 23 of the fiber 11. Roughly, the half-powerinteraction region at the plane of the grating is one-tenth as long andas wide as the side-polished region, so that the area of the half-powerinteraction region is about one-hundredth that of the side-polishedregion in the illustrated embodiment.

In the Bergh et al. reference cited above, the half-power region of asimilar half-coupler assembly was found to extend about 2.5 millimeters(mm) along the fiber 11 and between one and two core diameters, or sevenμm, transverse to the fiber 11. This very small transverse extentcontributes to the fact that the diverging ridges 19 can be consideredparallel for fixed positions of the grating 15, as discussed below.

As illustrated in FIG. 4, the grating 15 includes several regions A, B.C, D E and F. The multiple ridges 19 are shown gradually diverging sothat pitch increases as a function of transverse position. Thus, of theillustrated regions, region A has an intermediate pitch, region B hasthe smallest pitch, and region C has the greatest pitch.

The many regions, including those not separately illustrated, define acontinuum extending transversely of the fiber 11 in the Y direction.Thus, regions overlap, as shown in conjunction with regions D, E and F.Thus, by translating the grating 15 in the Y direction and therebyselecting the grating region to be positioned over the half-powerinteraction region of the fiber, the periodicity experienced by lighttransmitted along the fiber is changed.

Since the divergence of the ridges 19 is continuous, the tuning functionis a monotonic one-to-one mapping of grating position to reflectedwavelength. Alternatively, step functions and non-monotonic functionsare provided for.

Note that while the ridges 19 are clearly diverging, the segments 31aand 31b of ridges 19a and 19b within region A are very nearly parallel.FIG. 4 is necessarily schematic and cannot do justice to the degree ofparallelism. The degree of divergence is exaggerated by roughly a factorof a thousand in FIG. 4.

FIG. 4 also equates the regions A-F with projections of theside-polished region 23 of the fiber 11. However, it is more pertinentto equate the regions A-F with the half-power interaction region. Thewidth of the side-polished region is exaggerated by a factor of about32, so that the width of the half-power interaction is about 400 timesthinner than the width of the illustrated regions A-F. Parenthetically,FIG. 4 shows only about one of every 9000 grating lines.

Considering the exaggeration of divergence and the half-powerinteraction region together, the degree of parallelism is about 400,000times greater than indicated in FIG. 4. Thus, the geometry of thegrating provides for highly parallel ridge segments in the half-powerinteraction region, while permitting sufficient divergence for broadbandtuning, as detailed below after the following description of the methodused to manufacture the illustrated grating 15.

A grating such as grating 15 can be fabricated on a fused quartzsubstrate, which can be one inch square with one surface polished flat.The fused quartz substrate is used for the grating to prevent energyloss of the guided optical signal due to coupling to radiation modes inthe substrate. This is accomplished since the refractive index of thefused quartz is lower than the effective index of the guided mode.

A thin film of amorphous silicon can be sputtered onto the polishedsurface; the film can be about 0.3 μm thick. Silicon is used for thegrating structure since it has a large refractive index and hasrelatively low absorption losses at the infrared wavelengths, 13,000 to15,500 Å, of interest. The large index of refraction difference betweenthe amorphous silicon and the fused quartz contributes to an increasedreflection coefficient.

Photoresist can then be spun onto the amorphous silicon. The photoresistcan be a positive photoresist such as Shipley AZ1350, but otheralternative positive and negative photoresists can be accommodated. Thephotoresist layer can be about 0.4 μm thick.

The photoresist coat can then be exposed holographically using a largelyconventional apparatus 33 illustrated in FIG. 5. A holographicinterference pattern is initiated by a laser source 35, which in theillustrated embodiment is a Helium-Cadmium, HeCd, laser with a nominalwavelength of 4416 Å. A half-silvered mirror 37 is used to split thelaser beam 39 into two substantially equal and mutually perpendicularcomponent beams 41.

Respective mirrors 43 are then used to redirect the component beams sothat the redirected component beams 45 define a predetermined angle withrespect to a point of virtual convergence 47. The point of convergence47 is virtual in that respective lenses 49 and diffraction pin-holes 51disturb the redirected component beams 45 before they reach the would-bepoint of convergence.

The angle of convergence is selected to provide a standing waveinterference pattern. The appropriate angle is determined as a functionof the laser wavelength as is well-known in the art. In the illustratedembodiment, the 4416 Å wavelength of the HeCd laser 35 requires a 59°angle of convergence to generate the desired holographic interferencepattern.

The redirected component beams 45 are focused at their respectivepin-holes 51 by respective lenses 49. The pin-holes 51, which in theillustrated apparatus are about 2.5 μm in diameter, serve as coherentpoint sources of spherically diverging wavefronts. As is well known, thewavefronts thus created interfere to produce an interference patternwhich is sharply defined at the point of virtual convergence 47.

As in the fabrication of a conventional holographic grating, the coatedsubstrate is positioned about the point of convergence 47. However, inaccordance with the present invention, the substrate is exposed to awavefront generated by spherically diverging beams and is tilted about30° with respect to the wavefront interference plane.

To establish a frame of reference, let the point of convergence 47 be anorigin, with a line bisecting the 59° angle defining the x-axis; they-axis is then as illustrated in FIG. 5, and the z-axis is orthogonal tothe page. Diverging beams, rather than the conventional collimatedbeams, are used to generate a diverging interference pattern. The degreeof divergence at the substrate can be adjusted by rotating the it fromthe Yz-plane of the interference front as indicated in FIG. 6

This tilt causes a lower portion of the substrate to be nearer thepin-holes and an upper portion to be further from the pin-holes. Sincethe interference pattern diverges away from the pin-holes, a diverginginterference pattern is imposed on the photoresist. Those skilled in theart understand that the interference lines are hyperbolic rather thanstraight. However, straight lines are sufficiently approximated not onlyover the width of the half-power interactions regions, but also over thetuning range of the grating.

The strips of photoresist exposed to lines of constructive interferenceare then removed from the substrate by conventional methods. Thephotoresist pattern is then transferred to the amorphous silicon using aselective reactive ion etch. As gas such as carbon tetrafluoride can beused in the etch. Finally, the remaining photoresist is removed. Theresulting grating 15 has a spatial period of approximately 0.45 μm witha divergence of about 3.4 microradians.

The resulting grating 15 is then mounted on the xyz-translation stage17, and then placed ridges down onto the side-polished optical fiber 11to allow for evanescent interaction. An index matching oil 53 ispreferably applied between fiber 11 and grating 15 to remove anypossible air gaps which would reduce the extent of the evanescent field.

The grating 15 is longitudinally centered on the side-polished region 23of the fiber 11 with its ridges 19 extending substantially transverselyof the fiber 11. The x-control 55 of the translation stage 17 is used tocenter the grating 15 in the x-direction. The z-control 57 of thetranslation is adjusted to apply sufficient pressure to ensure firmcontact between grating 15 and the fiber 11. As indicated above, they-control 21 is used for tuning.

The block and fiber assembly can be fabricated as provided by Bergh etal., cited above. For completeness, the process is outlined herein.Since, Bergh et al. use two such assemblies to constitute a directionaloptical coupler, the block and fiber 11 can be referred to collectivelyas a "coupler half".

The coupler half includes the block 13 and the side-polished fiber 11.The groove 29 is cut or etched into the flat top surface 59 of the block13 between two end surfaces 61 such that the depth of the groove 29 withrespect to the top surface 59 is greater near the two end surfaces 61that it is midway between the two end surfaces 61. Preferably the depthof the groove 29 varies gradually such that the groove 29 is arcuatebetween the two end surfaces 61 as shown in FIG. 3.

The fiber 11 is preferably a standard single-mode telecommunicationsfiber used, for example, for the 13,000 to 15,500 Å wavelengths. Thefiber 11 has an inner core 25 and an outer cladding 27. The inner core25 has a higher refractive index than the outer cladding 27 so thatlight propagating within the inner core 25 of the optical fiber 11 isguided. The diameter the core 25 of the illustrated fiber 11 is 10 μm,while the cladding 27 has a diameter of 125 μm.

The fiber 11 is placed within the groove 29 with the axis of the opticalfiber 11 extending between the two end surfaces 61. The depth profile ofthe groove 29 is selected to be greater at the end surfaces 61 than thediameter of the outer cladding 27 of the optical fiber 11. The depth ofthe groove 29 midway between its ends is selected so that it isapproximately equal to the diameter of the outer cladding 27 so that thefiber 11 extends about to the surface of the block 13 at its midpoint.

The optical fiber 11 is held in place in the groove 29 by an epoxy orother suitable adhesive. Thereafter, the top surface 59 of the block 13and any coplanar portions of the cladding 27 are carefully ground andpolished so that a portion of the cladding 27 of the optical fiber 11 isslowly thinned. The grinding and polishing is continued until only asmall thickness of the cladding 27 covers the inner core 25 of theoptical fiber 11 at the approximate mid-point of the block 13.

For example, given the dimensions of the illustrated optical fiber 11,e.g., an inner core 25 with a diameter of 10 μm and a cladding 27 with adiameter of 125 μm, a portion of the cladding 27 is thinned untilapproximately 0.5-5 μm of the cladding 27 covers the inner core 25 atthe longitudinal midpoint of the groove 29. After the grinding andpolishing is completed, the region where the cladding has been removeddefines the oval side-polished region 23 which is coplanar with the topsurface 59 of the quartz block 13.

The performance of the illustrated filter 10 was evaluated bycharacterizing the spectral distribution of the reflected frequencies.Reflected wavelength is shown as a function of grating position in FIG.6. The dots indicated the empirical results of measuring reflectedwavelengths at different grating positions.

The diagonal line represents a least squares fit, as is known in theart. The proximity of the empirical points to the least squares lineindicates a high degree of tuning linearity. Apparently, the hyperbolicnature of the holographic interference pattern does not detract severelyfrom the linearity of the filter. The slope of the diagonal line is 96.8Å/mm, which corresponds to the 3.4 microradian divergence of the ridges19.

Graphs of power distributions over grating positions taken at differentincident wavelengths are shown in FIGS. 7A, 7B and 7C. The illustratedmaxima correspond very roughly with regions A, B and C in FIG. 4.

The graph of FIG. 7A was generated using an incident beam with awavelength of about 13,010 (Å). Maximum power is achieved around agrating position of 1.0 mm along the approximately 2.6 mm effectivetransverse dimension of the grating 15. The half-power bandwidth of thepeak is about 5.6 Å. This corresponds to a 57 μm translation of thegrating 15.

The graph of FIG. 7B was generated using an incident wavelength of about12,928 Å. Here the reflected power peak occurs at about 0.08 mm alongthe operating dimension of the grating 15. The half-power bandwidth isabout 6.7 Å, corresponding to a 69 μm movement of the grating 15.

The graph of FIG. 7C was generated using an incident wavelength of about13,153 Å. The reflected power peak occurs at about 2.4 mm alongoperating dimension of the grating 15. The half-power bandwidth is about5.6 Å, corresponding to a 58 μm translation of the grating 15.

Collectively, the three graphs of FIGS. 7A-C indicate a tuning range inexcess of 220 Å with a maximum reflected half-power bandwidth of 6.7 Å.This corresponds to a tuning-to-bandwidth ratio of about 33:1. Thiscompares very favorably with the 2:1 or poorer tuning-to-bandwidthratios provided by the infiber filters of the background art discussedabove.

In addition to the single-mode fiber, reflective filter embodimentdescribed above, multi-mode fiber transmissive filter embodiments areprovided for by the present invention. Multi-mode fibers include singlecore fibers with cores designed to support multiple transmission modeswithin a predetermined bandwidth. Alternatively, multiple modes can beprovided in a fiber having multiple modes supported by multiple cores,each core supporting a single mode.

Specifically, in a fiber supporting two propagation modes, each with acharacteristic velocity, a grating with spatial periodicity equal to the"beat" length for the two modes at a given frequency serves to couplethe modes. Thus, a relatively broadband signal along one of the modescan be selectively coupled to the other mode at the grating. Thewavelength coupled can be adjusted as above by moving the grating so asto change the spatial periodicity of the grating.

In the case of an optical fiber having two cores with differentgeometries and/or indexes of refraction so that they support differentsingle modes, a signal input into one core at one end of the fiber canbe selectively coupled into the other core at a grating filter. Thelight withdrawn from the second core at the opposite end of the fiberrepresents transmissively filtered light. The same filtering can beapplied to a multi-mode single core fiber, although the separation ofthe modes at the fiber output may be more complex.

All of the multi-mode embodiments just discussed are adequatelyrepresented by FIGS. 1-4. Of course, P_(R) must be taken out oppositethe end P_(IN) is introduced. In the case of a dual-core fiber, thecores are preferably parallel and transversely disposed with respect toone other, so that each can conveniently interact with the grating.

While the foregoing represent preferred embodiments, it is understoodthat many variations and modifications are also provided for by thepresent invention. For example, a grating 70 with an alternative patternis indicated in FIG. 8. Here, parallel ridges are arranged on agradually increasing pitch as a function of longitudinal position withrespect to a fiber. For example, the pitch of ridges 71 at onelongitudinal end of the grating 70 is less than the pitch of the ridges73 at the opposite longitudinal end; intermediate series of ridges haveintermediate pitches. Thus the periodicity at a half-power interactionregion of the fiber can be controlled by longitudinal translation of thegrating 70 along the x-dimension.

There are two basic variations of the embodiment of FIG. 8. In onevariation, the periodicity is varied step-wise and the longitudinaltranslation is performed in discrete steps. Thus, there is nominally nopitch variation over the interaction region.

Alternatively, the pitch can be varied continuously and continuousadjustment provided for by continuous translation. In this case, thepitch must be increased gradually enough so that negligible variationoccurs over the region of interaction with the evanescent field

Many other grating patterns are provided for. Radial gratings can befabricated with the radial pitch gradually increasing withcircumferential position. Such a grating can be rotated about an origindisposed transversely of the interaction region to provide variableperiodicity at the evanescent field.

Another grating includes circumferential ridges of radially varyingpitch. By rotating such a grating about an off-center point mountedlongitudinally of the center of the interaction region, the spatialperiodicity at the evanescent field can be controlled.

In addition, different methods of fabrication, wavelengths, fibergeometries and compositions, and methods of accessing the evanescentfield are provided for. In particular, the present invention applies toa filter subassembly to be attached to a existing fiber which is thenthinned for access to its evanescent field. Alternatively, the filterassembly can include a pre-polished fiber to be spliced into a fiberoptic system. These and other variations and modifications are providedfor by the present invention, the scope of which is limited only by thefollowing claims.

What is claimed is:
 1. A filter for use with optical fibers, said filtercomprising:a grating including plural elongated regions and pluralridges, said elongated regions sharing a common predetermined width,said elongated regions sharing a common predetermined length, eachregion intersecting plural of said ridges to define a series of ridgesegments, each series so defined extending lengthwise of the definingregion, each ridge segment of each said series extending substantiallywidth-wise across the region, each region being characterized by asubstantially constant ridge-segment pitch, with a first region having adifferent pitch from at least a second region; coupling means forarranging said grating with respect to an optical fiber so that theevanescent filed of light being transmitted through said fiber cansubstantially interact with said grating, and so that the intersectionof said grating and such an evanescent field can be substantiallycontained within at least one of said regions; positioning means formoving said grating relative to said fiber so that at one position saidfirst region can substantially contain the intersection of said gratingand the evanescent field of light being transmitted through said fiber,and so that at another position said second region can substantiallycontain the intersection of said grating and the evanescent field oflight being transmitted through said fiber; and an optical fiber segmentcoupled to said grating by said coupling means, said optical fiberincluding a single core and supporting a single mode in a givenwavelength band, said grating being arranged to interact with theevanescent field of light being transmitted in a first direction alongsaid core so that a portion of said light is reflected back along saidcore in the opposite direction.
 2. An apparatus for selectivitycontrolling the propagation of light in an optical fiber, said apparatuscomprising:an optical fiber extending between first and second ends andhaving an inner core and an outer cladding, a portion of said outercladding being thinned at a location intermediate to the ends of thefiber to form a facing surface thereon through which the evanescentfield of light being transmitted along the core of said fiber canemanate; a grating including gradually diverging ridges which extendsubstantially transversely to said optical fiber for introducing avariation in refractive index as a periodic function of longitudinalposition along said thinned portion of said outer cladding, theperiodicity of said grating at said thinned portion being a function ofthe relative position of said grating and said thinned portion; couplingmeans for coupling said grating to said thinned portion so that saidgrating can interact with the evanescent filed of light beingtransmitted along the core of said fiber; and; positioning means forvariably positioning said grating relative to said thinned region; saidoptical fiber including a single core and supporting a single mode in agiven wavelength band, said grating being arranged to interact with theevanescent filed of light being transmitted in a first direction alongsaid core so that a portion of said light is reflected back along saidcore int he opposite direction.
 3. An apparatus for selectivitycontrolling the propagation of light in an optical fiber, said apparatuscomprising:an optical fiber extending between first and second ends andhaving an inner core and an outer cladding, a portion of said outercladding being thinned at a location intermediate to the ends of thefiber to form a facing surface thereon through which the evanescentfield of light being transmitted along the core of said fiber canemanate; a grating including gradually diverging ridges which extendsubstantially transversely to said optical fiber for introducing avariation in refractive index as a periodic function of longitudinalposition along said thinned portion of said outer cladding, theperiodicity of said grating at said thinned portion being a function ofthe relative position of said grating and said thinned portion; couplingmeans for coupling said grating to said thinned portion so that saidgrating can interact with the evanescent field of light beingtransmitted along the core of said fiber; positioning means for variablypositioning said grating relative to said thinned region; and; an oilapplied between said ridges and said thinned portion of said fiber. 4.The apparatus of claim 3 wherein said oil has a refractive indexsubstantially equal to that of said cladding.